This invention relates generally to battery charging systems and more particularly to battery charging systems in which only a single cell of a battery is monitored to control the charging thereof.
Battery charging systems which sense predetermined conditions of a battery to control the charging thereof are well known. These include, battery charging systems which sense a fully charged or overcharged condition by checking overall battery bank terminal voltage, examples of such systems are shown in U.S. Pat. Nos. 3,296,515, 3,781,631, 4,061,956 and 4,146,830. Many rechargeable battery types have significantly lower terminal voltages when discharged. Undetected battery faults, such as deteriorated or shorted cells in systems which detect battery conditions by sensing terminal voltage may cause overcharging of the remaining cells without permitting a sufficient increase in the bank terminal voltage to actuate an overcharge sensing circuit. Therefore, in battery charging systems which base the charging function on overall battery bank terminal voltage, shorted cell sensing takes on added significance.
Shorted or defective cell sensing for many battery chargers, including those disclosed in the above referenced patents, and others such as described in U.S. Pat. Nos. 3,296,515 and 3,781,631, is accomplished by sensing low voltage at the battery terminals and discontinuing charging to the connected battery if the terminal voltage is lower than a predetermined normal rating. Some charging systems, such as that shown in U.S. Pat. No. 3,296,515 also include circuitry to override this feature by use of a test button which must be held on to force charging. However, larger batteries, which are often deeply discharged, may not regain rated voltage quickly under normal charging conditions.
Other battery charging systems detect defective cells by utilizing a voltage divider for sensing a moderate voltage imbalance between the two halves of the battery; see, for example, the above referenced U.S. Pat. No. 4,061,956. Such an arrangement, may, however, be ineffective if cell deterioration is approximately equal in each half of the battery.
Still other shorted or defective cell sensing battery charging systems include circuitry for continuous or periodic checking for abnormally high charger output currents in a constant voltage mode or timing a constant current charging mode to detect a significantly longer charging period than would normally be required to fully charge the connected battery. Either of these systems can shut down the charging circuit if battery faults are detected. However, both systems require a potentially damaging time delay to verify that a fault condition exists before interrupting the charging circuit.